1. Field of the Invention
The present invention relates to an optical modulation method wherein, in an optical transmission path using an optical fiber, intensity modulation of light propagating in the optical fiber can be externally performed in proportion to an input electrical signal without cutting the optical fiber. The present invention further relates to an optical modulator using the optical modulation method, and an optical telephone using the optical modulator.
2. Description of the Related Art
Optical fiber cables are installed in main trunk routes nationwide as transmission media suitable for a long-distance, large-capacity transmission system. For this reason, installation works of optical fiber cables and cable conversion works are increasing in number year by year. In works associated with optical fiber cables, workers at separate places, e.g., a repeater station and a manhole, do their work concurrently in many cases. In order to smoothly do their work, talking between the workers must be easily performed.
A metallic pair for communication in an optical fiber cable has been used for talking between the workers. However, as the length of an optical transmission path is increased, talking through a metallic pair cannot be performed. In addition, since a non-metallic optical fiber cable has been introduced to realize a non-inductive, lightweight optical fiber cable, talking through an optical fiber is required. In a conventional scheme, talking through an optical fiber is performed as follows. An optical fiber 1 is cut at cutting points 23 located at intermediate points, as shown in FIG. 1, and O/E converters (optical/electrical converters) 18, and E/O converters (electrical/optical converters) 17 are connected between the cut fibers through fusion splice points 24, as shown in FIG. 2. In each electrical section, demodulation and modulation are respectively performed by a demodulation circuit 20 and a modulation circuit 19. In this method, however, cutting of optical fibers, fusing of cords having optical connectors, mounting of optical connectors, and the like require a long period of time, resulting in poor operational efficiency. In addition, since an optical fiber is cut and an end-face treatment is performed every time talking is performed, the optical fiber is gradually shortened. As a result, the designed slack in the optical fiber in a closure may become too short. Therefore, demand has arisen for a talking method which allows talking at an arbitrary place without cutting an optical fiber. As conventional talking methods allowing talking at an arbitrary place without cutting an optical fiber, methods of using local injection for transmission are known, e.g., methods disclosed in (1) Published Unexamined Japanese Patent Application No. 2-21735 (corresponding to an optical output section in FIG. 3) and (2) Electron. Lett. vol. 20, no. 3, pp. 109-110 (1984). In addition, methods of using local detection for reception are known, e.g., methods disclosed in (1) IWCS (International Wire & Cable Symposium Proceedings) 1989, and (2) J. Lightwave Technol., LT-5, 12, pp. 1663-1665 (1987).
In the local injection method, as shown in FIG. 3, a modulation signal is transmitted from an input signal source 6 to a light-source 21 to radiate modulated light 22 from the outside of an optical fiber 1 onto its bending region, thus forcibly injecting the light into the core of the optical fiber 1. The local detection method is a method of receiving light radiated from an optical fiber bending region.
In the local injection method, however, since the coupling efficiency of light radiated from the outside of the optical fiber into the optical fiber core is low, a signal can only be transmitted several kilometers at most. In addition, it is difficult to stably supply high radiation power. For these reasons, the local injection method is not very practical as a transmission method, and hence another transmission method is required.
Furthermore, a method of performing intensity modulation of light propagating in an optical fiber by applying/removing a bending force to/from the optical fiber has been proposed (see Published Unexamined Japanese Patent Application No. 64-35504). In this method, light propagating in an optical fiber is subjected to only intensity modulation in a digital manner. However, a method capable of analog modulation for, e.g., a voice signal is preferable.
In an apparatus capable of a talking operation without cutting an optical fiber, as disclosed in, e.g., 1990 Spring Natl. Conv. Rec. IEICE, B-904, vibrations are applied to an intermediate point of an optical fiber from its side surface by using a piezo-electric ceramic to cause fluctuations in polarization state of linearly polarized light propagating in the optical fiber. When the fluctuations in polarization are detected by a polarizer at a center, changes in intensity of the light can be obtained, and a signal can be obtained. This method allows modulation without cutting an optical fiber. However, talking between intermediate points must be performed as follows. Signal light transmitted from one intermediate point is demodulated at the center. Intensity modulation of a light-source is performed by using the resulting electrical signal. The resulting light is transmitted on a different optical fiber. The signal light is then detected at the other intermediate point by the local detection method, thus performing a talking operation. That is, the method is not very practicable, requiring two optical fibers for a talk, and allowing only a short-distance talk. Moreover, according to a method disclosed in Published Unexamined Japanese Patent Application No. 2-291517, a coil-like bending region is formed, and light propagating in an optical fiber is modulated by changing the coil bending radius. In this method, however, since the bending radius is changed by applying a tension to the optical fiber in the longitudinal direction, a frictional resistance is produced between a fiber support portion and the optical fiber. As a result, the optical fiber is distorted to cause a deterioration in modulation characteristics. The absence of a mechanism for adjusting the bending radius is another factor contributing to the deterioration in modulation characteristics.